Process for the production of sheet and strip from ferritic, stabilized, stainless chromium-molybdenum-nickel steels

ABSTRACT

PCT No. PCT/EP78/00011 Sec. 371 Date Apr. 17, 1979 Sec. 102(e) Date Apr. 17, 1979 PCT Filed Aug. 16, 1978 PCT Pub. No. WO79/00100 PCT Pub. Date Mar. 8, 1979 
     A process for the production of sheet an strip from ferritic, stabilized, stainless chromium-molybdenum-nickel steels by hot-rolling of cast blocks to form heavy plate or crude strip and subsequent cold-rolling to the required thickness, wherein the blocks are hot-rolled at temperatures above about 850° C. to form heavy plate or crude strip and, immediately afterwards, the heavy plate or crude strip thus formed is suddenly (rapidly) quenched with water to a temperature just below 450° C. and subsequently annealed, the annealing step being followed by sudden (rapid) cooling from temperatures above about 850° C. to temperatures below 500° C.

DESCRIPTION

In the production of sheet and strip from stainless steels by rolling,the block is heated to the rolling temperature and rolled in severalpasses, intermediate treatments and, in particular, surface treatments,such as grinding, generally having to be inserted, which can only bedone at temperatures of at most 300° C. Surface treatment in slab formis particularly important in this respect.

It has now been found that, despite an extremely good surface quality ofthe block, ferritic chrome-molybdenum-nickel steel with nickel andmolybdenum contents of up to about 5% is particularly unsuitable forintermediate treatments of the type in question because the material isdestroyed by sudden hardening phenomena and cracking. This tendencytowards hardening and cracking is particularly pronounced in crudeplate, i.e. in heavy plate and crude strip having a thickness of fromabout 3 mm to about 8 mm and, in particular, around 5 mm. It isreflected inter alia in the fact that a strip rolled into a coil cannotbe unrolled because it is too hard and, at the same time, containscracks.

The object of the present invention is to provide a process whichobviates these disadvantages and gives satisfactory sheet and stripmaterial with favourable properties.

According to the invention, this object is achieved by a process whichis characterised in that the cast blocks are hot-rolled at temperaturesabove about 850° C. to form heavy plate or crude strip and, immediatelyafterwards, the crude strip thus obtained is suddenly (rapidly) quenchedwith water to a temperature just below 450° C. and is subsequentlyannealed, annealing being followed by sudden (rapid) cooling fromtemperatures above about 850° C. to temperatures below 500° C., theheavy plate or crude strip having a thickness of from about 3 to 8 mmand, in particular, around 5 mm.

The invention shows how this relatively expensive steel can be processedby rolling and under certain conditions of heat treatment and how itretains its extremely favourable properties in its final strip form.

According to the invention, the intermediate products are not exposed atany stage of the entire rolling process to gradual cooling totemperatures from above about 850° C. to just below 450° C., whichapplies in particular to the condition after finish-rolling. Since thissteel shows particularly good hot forming properties, the material isprevented from falling below the upper temperature limit of 850° C. andfrom slowly passing through the temperature range from 850° C. to 450°C. by directly rolling down the block in a single operation to form theheavy plate or crude strip, the final temperature having to be above850° C. from which the heavy plate or crude strip is suddenly (rapidly)cooled to below 450° C. The thickness of the heavy plate or crude strippreferably amounts to between about 3 mm and about 8 mm andadvantageously to approximately 5 mm.

The process according to the invention may be applied with advantage tochrome-molybdenum-nickel steels having the following composition:

0.01 to 0.025% of carbon

0.005 to 0.025% and preferably 0.005 to 0.015% of nitrogen

22.0 to 27.0% of chromium

3.0 to 5.0% of molybdenum

3.2 to 4.8% of nickel

0.02 to 1.0% of manganese

0.02 to 1.0% of silicon

at most 0.25% each of vanadium, tungsten, cobalt and aluminium

0.1 to 1.0% of copper

0.2 to 0.7% of titanium and/or

0.2 to 1.0% of niobium,

the rest consisting of iron with the usual impurities and alloyingadditions of boron and/or zirconium being permitted in contentscorresponding to the prior art.

The invention will now be explained in more detail with reference to anexample. A block weighing 10 t, which had been melted and cast by theAOD method, had the following composition:

0.012% of carbon

0.4% of silicon

0.32% of manganese

25.7% of chromium

4.2% of nickel

4.08% of molybdenum

0.45% of titanium

0.55% of copper

0.059% of aluminium

0.011% of niobium

0.015% of nitrogen

rest iron.

This block was preheated and introduced into a reheating furnace inwhich it was heated to temperatures of from about 1150° C. to about1250° C. and, in particular, to a temperature of about 1200° C. Theblock was then introduced into the hot rolling stand and, in a singleoperation, was rolled down to a thickness of about 5 mm directly, i.e.without interruption. According to the invention, the strip emergingfrom the hot rolling stand was suddenly (rapidly) quenched with waterfrom a temperature above about 850° C. to a temperature of 420° C. Thestrip was then subjected to an annealing treatment which alsocharacterises the invention in that annealing was followed by sudden(rapid) cooling from temperatures above about 850° C. to temperaturesbelow 500° C., preferably around 400° C., after which the strip waspickled and rolled into a coil which was introduced into the coldrolling stand.

In the production of steel of the same quality without thecharacteristics of the present invention, the block was hot rolled intoslab form, i.e. to a thickness of 150 mm, and was then slowly cooled.The slab surface was fault-free in the hot state. When the slab wasre-examined after complete cooling, such serious cracking was found thatthe slab could not be used for further processing.

In another case, steel of the same quality was again melted, butdirectly rolled down to approximately 5 mm thick strip, followed by slowcooling. While still hot, the strip was rolled into a coil. However, thestrip could not be coiled again after cooling because it had becomeextremely hard and brittle. At the same time, serious cracking was foundso that the entire strip had to be discarded as scrap.

Another very important measure in the practical application of theprocess according to the invention lies in the fact that theintermediate annealing treatment, particularly between the individualcold-rolling passes, should be carried out at a temperature around 1025°C. which preferably should not deviate by more than about ±25° C. ineither direction.

In addition, it is also necessary during welding of the sheets andstrips thus produced to pass through temperature ranges which, accordingto the invention, should be avoided. Accordingly, provision should alsobe made for rapid cooling in cases where low cooling rates between 850°C. and 450° C. can occur. This applies in particular to the welding ofrelatively thick sheet, but is not necessary in the welding of sheetsless than 2 mm thick. However, whether heating is intentional orunintentional, the sheets will always be heated to around 1025° C. andrapidly cooled to a temperature from above about 850° C. to below about450° C.

We claim:
 1. A process for the production of sheet and strip fromferritic, stabilised, stainless chromium-molybdenum-nickel steels whichare hot-rolled in the form of cast blocks to form heavy plate or crudestrip and subsequently cold-rolled to the required thickness,characterised in that the blocks are hot-rolled at temperatures aboveabout 850° C. to form heavy plate or crude strip and immediatelyafterwards the heavy plate or crude strip thus formed is suddenlyquenched with water to a temperature slightly below 450° C. and is thensubsequently annealed to a temperature above 850° C., the annealing stepbeing followed by rapid cooling from temperatures above about 850° C. totemperatures below 500° C.
 2. A process as claimed in claim 1,characterised in that the hot-rolled heavy plate or crude strip has athickness of from about 3 mm to about 8 mm.
 3. A process as claimed inclaim 2, characterised in that the hot-rolled heavy plate or crude striphas a thickness of about 5 mm.
 4. A process as claimed in claim 1 or 2or 3, characterised in that the hot rolling of the blocks to form heavyplate or crude strips is performed at a starting temperature of fromabout 1150° C. to about 1250° C. in a single operation withoutintermediate annealing, and wherein cold rolling is performed as thefinal step, with intermediate annealing being carried out betweenindividual cold rolling passes at a temperature of from about 1000° toabout 1050° C.
 5. A process as claimed in claim 4, characterised in thatthe blocks are hot-rolled at a starting temperature of around 1200° C.6. A process as claimed in claim 1 or 2 or 3 or 5, characterised in thatthe starting material used is a chrome-molybdenum-nickel steel whichconsists of 0.01 to 0.025% of carbon, 0.005 to 0.025% of nitrogen, 0.02to 1.0% of manganese, 0.02 to 1.0% of silicon, 22.0 to 27.0% ofchromium, 3.0 to 5.0% of molybdenum, 3.2 to 4.8% of nickel, 0.1 to 1.0%of copper, 0.2 to 0.7% of titanium and/or 0.2 to 1.0% of niobium and ofvanadium, tungsten, cobalt and aluminum in maximum proportions of 0.25%in each case, the rest consisting of iron with the usual impurities,alloying additions of boron and/or zirconium being permitted.
 7. Aprocess as claimed in claim 4, characterised in that the startingmaterial used is a chrome-molybdenum-nickel steel which consists of 0.01to 0.025% of carbon, 0.005 to 0.025% of nitrogen, 0.02 to 1.0% ofmanganese, 0.02 to 1.0% of silicon, 22.0 to 27.0% of chromium, 3.0 to5.0% of molybdenum, 3.2 to 4.8% of nickel, 0.1 to 1.0% of copper, 0.2 to0.7% of titanium and/or 0.2 to 1.0% of niobium and of vanadium,tungsten, cobalt and aluminum in maximum proportions of 0.25% in eachcase, the rest consisting of iron with the usual impurities, alloyingadditions of boron and/or zirconium being permitted.
 8. A process asclaimed in claim 1 or 2 or 3, wherein intermediate products formedduring the process are prevented from being exposed to gradual coolingfrom temperatures above about 850° C. to just below 450° during theentire hot-rolling step.
 9. Process for the production of strip fromferritic, stabilised, stainless chromium-molybdenum-nickel steels withnickel and molybdenum contents of up to about 5%, wherein the cast heavyplate is hot-rolled to a thickness of about 5 mm and the crude stripproduced in this way is subsequently cold-rolled to the requiredthickness, the hot-rolled crude strip being rolled into a coil andunrolled again when being cold-rolled, characterised in that the castheavy plate is hot-rolled, at a temperature of about 1200° C., in onepass to a thickness of about 5 mm and the crude strip produced in thisway, after having emerged from a hot rolling stand is immediatelyrapidly quenched with water from a final temperature above about 850° C.to a temperature slightly below 450° C., is subsequently annealed andthen is cold-rolled.
 10. Process as claimed in claim 9, characterised inthat the strip is pickled and rolled into a coil prior to said coldrolling.
 11. Process as claimed in claim 9, characterised in that theferritic, stabilised, stainless chromium-molybdenum-nickel steelsconsist of0.01 to 0.025% of carbon, 0.005 to 0.025% preferably 0.005 to0.015% of nitrogen, 22.0 to 27.0% of chromium, 3.0 to 5.0% ofmolybdenum, 3.2 to 4.8% of nickel, 0.02 to 1.0% of manganese, 0.02 to1.0% of silicon,vanadium, tungsten, cobalt, and aluminum in maximumproportions of 0.25% each, 0.1 to 1.0% of copper, 0.2 to 0.7% oftitanium and/or 0.2 to 1.0% of niobium,the rest consisting of iron withthe usual impurities, alloying additions of boron and/or zirconium beingpermitted.
 12. Process for the production of sheet from ferritic,stabilised, stainless chromium-molybdenum-nickel steels with nickel andmolybdenum contents of up to about 5%, wherein the cast heavy plate ishot-rolled to a thickness of about 5 mm and the crude sheet produced inthis way is subsequently cold-rolled to the required thickness,characterised in that the heavy plate is hot-rolled in one pass at atemperature of about 1200° C. to a thickness of about 5 mm and the crudesheet produced in this way is then removed from a hot rolling stand andimmediately rapidly quenched with water from a final temperature aboveabout 850° C. to a temperature slightly below 450° C., is thereafterannealed and then cold-rolled.
 13. Process as claimed in claim 12,characterised in that the annealing is performed at a temperature ofabove about 850° C. and directly followed by rapid quenching to atemperature of below 500° C., subsequently pickled and then cold-rolled,intermediate annealing being carried out at about 1000° C. to 1050° C.between the individual cold-rolling passes.
 14. Process as claimed inclaim 13, wherein the quenching directly following said annealing is toabout 400° C.
 15. Process as claimed in claim 12 or 13, characterised inthat the ferritic, stabilised, stainless chromium-molybdenum-nickelsteels consist of:0.01 to 0.025% of carbon, 0.005 to 0.025% preferably0.005 to 0.015% of nitrogen, 22.0 to 27.0% of chromium, 3.0 to 5.0% ofmolybdenum, 3.2 to 4.8% of nickel, 0.02 to 1.0% of manganese, 0.02 to1.0% of silicon,vanadium, tungsten, cobalt, and aluminum in maximumproportions of 0.25% each,
 0. 1 to 1.0% of copper,0.2 to 0.7% oftitanium and/or 0.2 to 1.0% of niobium,the rest consisting of iron withthe usual impurities, alloying additions of boron and/or zirconium beingpermitted.
 16. Process as in claim 1 or 8 or 12, wherein the quenchingimmediately following said hot-rolling is to a temperature of about 420°C.